A glass-ceramic body is composed of a myriad of fine-grained crystals of relatively uniform size which are randomly oriented and homogeneously dispersed in a glassy matrix. The crystals comprise the major portion of the body. Such bodies are produced from a corresponding glass body by heat treating the glass body to cause controlled crystallization.
The basic character of glass-ceramics, and their production by thermally induced nucleation and crystallization, are described in detail in U.S. Pat. No. 2,920,971, granted Jan. 8, 1960 to S. D. Stookey, and reference is made thereto for further discussion. The patent explains that the first step is to melt a glass-forming batch, usually including a crystallization catalyst or nucleating agent. The melt is then simultaneously cooled and a glass article of desired configuration shaped therefrom. This glass shape is thereafter heat treated to initially form nuclei which subsequently act as sites for the growth of crystals thereon as the temperature is raised and the heat treatment continued. Since crystallization occurs at the innumerable nuclei formed throughout the glass, the resulting crystals are, perforce, large in number, uniformly dispersed in the body, and fine-grained.
Glass-ceramic articles are highly crystalline, the crystal phase usually constituting over 50% by weight of the body, and frequently over 75%. Hence, the physical and chemical properties of the glass-ceramic body generally approximate those of the crystal phase or phases present rather than the original glass. The crystal phases developed are dependent upon the composition of the original glass and the heat treatment applied thereto. Thus, it is often possible to cause the crystallization of one particular phase at a low temperature and a different or additional phase at higher temperatures. Inasmuch as the crystallization occurs in situ, glass-ceramic bodies are free of voids and non-porous.
In the customary commercial manufacture of glass-ceramic articles, a glass-forming batch is melted; the melt is cooled to at least within, and normally below, the transformation range to form a glass body; and the glass body is then reheated to cause crystallization in situ. This heat treatment conventionally comprises two steps. First, the glass body is heated to a temperature within or somewhat above the transformation range to cause the development of nuclei in the glass. This process commonly requires about 1-6 hours. Subsequently, the nucleated body is heated to a higher temperature, frequently above the softening point of the glass, to effect the growth of crystals on the nuclei. This step normally involves about 1-8 hours.
The time required to carry out this customary commercial practice, while seemingly quite long, has generally been considered necessary to insure production of uniformly crystalline bodies exhibiting little dimensional change during the conversion from glass to glass-ceramic. It is obvious that such lengthy processing is rather expensive at best, and that such extended times are completely imcompatible with certain products and processes.
For example, it is recognized that glass-ceramic fibers could be very useful, especially for such purposes as concrete reinforcement, but lengthy processing times are incompatible with normal fiber drawing processes. It has also been recognized that glass-ceramic coatings on either glass or metal filaments or strands can provide highly advantageous properties, but again it would be highly desirable to conduct one continuous processing operation. Such uses for glass-ceramic materials, and the advantages thereby attained, are described in greater detail in my copending applications Ser. Nos. 945,508 and 945,507, filed of even date herewith and entitled "Optical Waveguides" and "Basalt Glass-Ceramic Fibers and Method of Production."
As explained and illustrated in greater detail in the companion applications mentioned above, and in the literature cited therein, a commonly used method of continuous fiber or filament production involves drawing the fiber or filament continuously from a container of molten glass. Also, elongated glass and metal filaments or strands may be coated by drawing the elongated article through a bath of glass. In any case, it is readily apparent that the operation would be greatly simplified if the glass fiber, or coated filament, could be immediately exposed to a short heat treatment that would effect conversion to a glass-ceramic state as part of the drawing operation. Each of the companion applications discloses such an expeditious method.
A very important characteristic of any glass-ceramic material is the uniformly fine-grained nature of its crystalline phase, a property generally attributed to nucleated crystallization. In general, the strength of a glass-ceramic material, as measured by its modulus of rupture, is greater than that of the precursor glass. This is generally attributed to the nature of the crystal phase. In contrast, the well-known phenomenon of devitrification in glass, wherein spontaneous crystal growth occurs from the surface of a body into the interior, produces large, oriented, non-homogeneous crystals which result in a brittle material of relatively little strength.
A very fine grain size is especially important in resilient articles and coatings where at least one dimension is very small. The crystal size must of course be much smaller, else the article loses its resiliency, behaves physically like a true ceramic, and is very susceptible to physical damage.